To efficiently interact with exceedingly rich contents in natural vision, it is important for observers to retain precise memory representations that closely correspond to external stimuli in both visual short-term memory (VSTM) and visual long-term memory (VLTM). Evidence from animal and human memory research indicates that VLTM precision is supported by hippocampal pattern separation, a computation that orthogonalizes similar memories into distinct representations such that precise VLTM is possible in the face of interference. However, it is unclear what mechanisms underlie VSTM precision. The present study examines whether mnemonic precision of both VSTM and VLTM is supported by hippocampal pattern separation. Using an individual differences approach, Experiment 1 (n = 52) obtained behavioral evidence that VSTM precision, VLTM precision, and pattern separation are related. Specifically, behavioral measures of VSTM precision from a short-term color recall task, VLTM precision from a long-term color recall task, and pattern separation performance from a mnemonic similarity task were significantly correlated with one another, suggesting large shared variance among these constructs. Using fMRI, Experiment 2 (n = 19) directly linked VSTM precision to hippocampal activities. In this experiment, different demands on mnemonic precision were introduced by asking participants to memorize 4 colors randomly sampled from a color wheel with 180 colors (high-precision), 15 color spokes (medium-precision), or 6 color spokes (low-precision) – and to recall one of the studied colors from a corresponding color wheel. By comparing BOLD signals across these three precision load conditions and a perceptual and motor control condition, we tracked brain activities that increased with VSTM precision load. Specifically, we found that BOLD signals in the left hippocampus changed as a function of VSTM precision load, along with the medial prefrontal cortex and bilateral angular gyrus. Altogether, these findings support a shared computational and neural mechanism for mnemonic precision of visual memories.